Gypsum boards, also known as gypsum panels, drywall and wallboards, are popular construction materials with desirable properties for indoor applications. They are durable, economical and fire-retardant. In addition, these boards provide excellent compressive strength properties and a relatively low density. They are easily decorated and are attractive as surfaces, especially for interior construction.
Manufacturing of gypsum boards includes forming a slurry of calcium sulfate hemihydrate, water and additives and continuously depositing the slurry on a conveyor belt or a forming table. Often, a paper cover sheet, also known as a facer, moves on the conveyor beneath a mixer to continuously deposit slurry on the facer. Often, a second paper cover sheet, or facer, is applied over the slurry. The resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with water in the slurry, converting the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing the slurry to set and become firm. This forms a continuous strip of hardened material having, optionally, no cover sheets, a front and back cover sheet, or just one cover sheet on either the front or back of the panel. The continuous strip moves on the conveyor until the calcined gypsum is sufficiently set to withstand handling and movement from the conveyor to another place, such as a kiln, and the strip is thereafter cut to form boards of desired length. Water that is in excess of the amount needed for hydration of the calcined gypsum is removed from the gypsum panel in a kiln.
Gypsum panel manufacturers often use a biocide to protect the panels from attack by microorganisms such as mold and fungi by treating the paper coverings. However, treated paper alone is often insufficient to control mold growth for a number of reasons. Many biocides lose efficacy through the drying process in the kiln due to the high temperatures. The biocide can be overwhelmed by large quantities of mold spores that are incorporated into the gypsum and paper from water used during the panel forming process, combined with spores from the air. In some cases, per environmental regulations, there is a limit to the concentration of biocide that can be present on the surface of the paper. It appears that the maximum allowable biocide concentration is not sufficient to protect both the paper and the set gypsum core in all cases.
Microbial growth favors environments where spores find moisture and nutrients to metabolize. Temperature is also a factor, but numerous species of microorganisms thrive at the temperatures required for human habitation, where gypsum boards are most often used. Therefore, opportunities to control microbial growth consist mostly of controlling availability of moisture and nutrients. It is desirable to have a mechanism for killing microorganisms that begin to grow in or on a gypsum panel or a facer. Water vapor and spores are unavoidable in environments where gypsum panels are used, even though gypsum board is normally used in interior construction. In addition to moisture that is present in the environment, products used in interior construction sometimes encounter water due to seepage, leaky roofs or pipes, flooding, condensation, and the like. These exposures occur without any defects in the gypsum board manufacture or use. It is accepted that once exposed to moisture, traditional gypsum panel products are susceptible to microbial growth.
Starch is an example of a nutrient that microorganisms thrive on. In gypsum panels, starch is frequently used for a number of purposes. It may be added to a calcined gypsum slurry to promote adhesion between the core and the facer. Often a facer is made of paper, and starch may be a component of the paper commonly used to cover gypsum panels. Starch (sugar) coated particles of calcium sulfate dihydrate are often used as a set accelerator in a calcined gypsum slurry. Other starches may also be used to modify different properties of the set gypsum composition. When starches are present in the cover materials or the gypsum cores of gypsum panels, there is sufficient nutrition for possible microbial growth once the spores come into contact with the nutritious medium of the farinaceous panel.
Cover sheets for gypsum panels, also known as facers, facing material, paper facers, etc., are made by a paper manufacturing processes that begins with preparation of a dilute pulp of fibers, chemical additives and water. The pulp is drained through a screen to form a mat of randomly intertwined fibers. Additional water is removed by pressing the mat or applying suction. Informally, the “wet end” refers to the paper-making process before water removal, and the stage of the process after excess water is removed is called the “dry end.” Additives, such as size, may be added during either or both of these stages.
Paper size is a hydrophobic compound that improves a paper's strength and its resistance to penetration by liquids such as water and ink. Alkyl ketene dimer (“AKD”) and alkenylsuccinic anhydride (“ASA”), both of which are hydrophobic, are common sizing agents. Rosin and rosin derivatives are another class of paper sizing agents known in the paper industry. For good sizing efficiency, the size is applied as very small particles. This, and the hydrophobic property, requires that ASA and/or AKD be emulsified in an aqueous solution in order to properly introduce and anchor the sizing to the paper's fibers. Internal size is incorporated into the paper itself during the wet end of the manufacturing process. External size is applied to the surface of the finished paper product by dry end coating processes such as dipping, spraying or rolling.
ASA internal size is usually prepared on-site at a paper plant by emulsion with a cationic starch stabilizer as described in U.S. Pat. No. 6,159,339, herein incorporated by reference. A high charge, low molecular weight polymer may also be used as an emulsifier of the internal paper size in water. Alternatively, an AKD emulsion may be prepared by first dispersing a starch phosphate derivative in the water which is to become the continuous phase of the emulsion. Then, AKD is added and thoroughly admixed at temperatures from about 140° F.-160° F. until a smooth, homogeneous emulsion is attained. High shear mixing equipment is used to agitate the ketene dimmer and aqueous starch phosphate mixture to attain the desired emulsion.
Disadvantages associated with known AKD emulsification practices are typically overcome by emulsifying the AKD off-site and supplying it to paper manufacturers as a fully formulated emulsion. Emulsifying AKD is a difficult process that ordinarily requires expensive and highly specialized equipment. For the purpose of stabilizing AKD emulsions, additives such as surfactants and protective colloids may be present in the emulsion composition. The AKD may react with some of these additives, thereby reducing the efficiency of the size by reducing the amount of active ingredient that is available. Anionic surfactants present in an AKD emulsion further reduce efficiency of the size because the cellulosic material to which the size is expected to anchor is also anionic, thereby repelling the size particles rather than favoring introduction of the size to the cellulosic fibers. Another disadvantage of the typical AKD emulsion supply is economic because it is expensive to transport the large amounts of water that are part of the AKD emulsion to the paper manufacturer.
ASA emulsions are unstable, with a maximum shelf life between 6 and 8 hours depending on the make down water pH and temperature. Typically, the ASA emulsion is stored for 30 minutes prior to use. It is desirable to keep the ASA oil very dry and to wait until the last possible moment to prepare the aqueous emulsion. Frequently, paper-makers prepare the desired amount of ASA emulsion 30 minutes before the solution is added to the furnish. Cationic starch emulsifiers utilized in preparation of the aqueous ASA size emulsions provide a cationic starch sheath around each ASA droplet, anchoring the size to the anionic cellulosic paper fibers. It is still possible for much of the ASA to flow from the fibers with the process water. This gives the ASA time to decompose by hydrolysis, impairing the ASA sizing efficiency, causing deposit to form on the paper machine, higher operating costs and paper quality issues. Complicated and expensive wet end chemistry is often needed to achieve satisfactory retention of the size. Dry end testing, such as high performance liquid chromatography (“HPLC”) is common to ensure that retention of the ASA size is satisfactory and that it is consistent.
Prior art attempts to reduce microbial growth on gypsum boards include replacing paper facings with fiberglass-based facings, eliminating a source of starch nutrition and deterring microorganisms from growing on the board surfaces. Attempts to make gypsum boards resistant to microbial growth have also been made by incorporation of a biocide, such as a salt of pyrithione, into the core, the facers, or both, as revealed in U.S. Pat. No. 6,893,752 entitled “Mold Resistant Gypsum Panel and Method of Making Same,” herein incorporated by reference.
Quaternary ammonium compounds are loosely defined as a class of compounds generally having the formula R1R2R3R4—N+Y−, where the radicals may be the same, different or part of a ring and Y is a counter anion. Typically, but not always, one of the radicals is a long-chain alkyl group. Certain quaternary ammonium compounds possess biocidal properties. Prior art teaches the use of biocidal quaternary ammonium compounds in the gypsum core, or as a surface coating of paper facers, whether applied by spraying, dipping, rolling or any other dry end coating method.
While quaternary ammonium compounds are appreciated for their ability to control the growth of microorganisms, they are often avoided in paper-making because they produce foam, even at low concentrations. Foam has detrimental effects on the quality of the final paper product by forming pin-holes, circular marks on the paper, lower paper strength and reduced production. Often, the solution to foam problems involves complicated wet end chemistry to prevent foam formation with anti-foam compounds or to de-foam the paper furnish with de-foaming compounds. Another method of controlling foam in aqueous solutions of quaternary ammonium compounds is by adding anionic surfactants to the solution, as disclosed in International Publication Number WO 2008/049616 entitled “Controlled Foam Aqueous Quaternary Ammonium And Phosphonium Compositions,” herein incorporated by reference. As noted in this publication, the biocidal efficacy of quaternary ammonium compounds is compromised by addition of the anionic surfactant.
There is an ongoing need for gypsum board products that offer reduced susceptibility to microbial growth without compromising their beneficial properties. In addition, there is an ongoing need for commercially viable manufacturing methods for such products. There also remains a need for improvement in the efficiency and workability of AKD and ASA paper size as well as an improvement in retention of biocide compounds used in paper-making.